The storage and flow of mixtures of fluids and solids is widespread in industry. Such flows include pure gas, wet gas, pneumatic conveying of dry solids, slurry flows, sewage, and multiphase flows in the oil industry. Flows occur in many industries including power generation, water and waste treatment, oil and gas production, the food industry, mining, and the chemical industry. Consideration of flow is also important in applications such as aircraft with respect to gas flow through engines and indeed airflow passing an airframe or other components such as engine blades or ducts.
The multiphase mixture of materials to be measured may be flowing along a pipeline, or recirculating within a vessel, including for example fuel or propellant tanks in vehicles, aircraft, spacecraft or satellites.
The liquids may be water, as in wet steam flows, oil or gas condensate, or any other fluid that is liquid at the pressure and temperature of the pipe or pipeline within which it is contained. The phases, gas, solid and liquid, have a number of properties. They may be miscible—as in oil/gas or water/steam combinations, or immiscible as in water/gas or oil/air mixtures. The materials may be electrically conducting, as for example water, or non-conducting. The phases may be in thermodynamic equilibrium with mass interchange between the phases, as in water/steam or condensate/gas, or with no interchange such as when water is flowing in natural gas.
It is known that in pipeline flows or within storage tanks the flow may not have an even distribution. Only a fraction of the pipe or tank may be “occupied”. It is known in the art to consider the phases of the gas, liquid or solids entrained and to refer to a first phase and a second phase to differentiate between, for example, the water and the gas within which the water is dispersed. In this example, the water could be termed the first phase and the gas the second phase.
Pneumatic conveying is a further application where a fraction of the flow pipe is occupied by a dispersed second phase, in this case dispersed solids such as grains, minerals and powders are carried by an air or gas which is the first phase. The air or gas is pumped through the system to give the motive power to convey the solids which may flow as a sliding bed, in wave-like structures or dispersed in the flow.
Conveying of solids may also be undertaken by water or other liquids, sewage being a typical example.
Fluid flows which may include entrained liquid, gas or solids are thus widespread in many industries and with many different combinations of gas, solid and liquid. It should be understood that the invention described here applies to any flow or storage of gas or mixture of gas, solid and liquid in any or all of such industries and conditions, the examples above are not regarded as exclusive. The term industrial should not be taken to be limited to a process carried out for the purpose of manufacture or product delivery. It should be interpreted broadly to include the detection of parameters in fluid flows whether they are involved in an industrial process or not.
The fluids in a multiphase flow may be distributed in the direction of flow in many ways. For example, it may be distributed as droplets, bubbles, waves, or wisps within the main body of the flow; as a film on the surface of the pipe or as a continuous medium with the other phases distributed within. In all cases, the liquids or gases will normally be travelling at different velocities, with some phases moving less quickly than others with this being known in the art as “slippage”. The difference between a local gas velocity and a local liquid velocity is called the local slip velocity, while the difference between some average of the gas velocity and some average of the liquid velocity is known as the average slip velocity.
Similar conditions exist in conveying of solids in gases or liquids where the solids may be distributed across the flow in many forms at various densities and may be moving at a different velocity to the main flow. The flow is often quantified by reference to a mass flow rate that being the mass of substance which passes a point per unit of time.
The mass flowrate of gas in wet gas flows is normally of significant commercial importance, as, for example, in the transfer of natural gas or steam for sale, or in the management of the process, such as in power generation where the steam mass flowrate is the primary energy input to a turbine. The term wet gas is used here in a general sense to describe any flow that contains significant quantities of gas carrying with it some quantity of a liquid or mixture of liquids. The mass flowrate of liquid may also have a financial cash value, as in gas condensate flowing from a gas production unit, it may be important for technical management as in supply to a steam turbine, or it may have no particular financial value/importance, as in supply of steam for heating. The measurement of the flow may be required for taxation purposes referred to in the art as a “fiscal measurement”.
In many applications, it is not the mass flowrate of gas or liquid in the conveying system but the flowrate of the dispersed solid that is of critical interest to the operator of the process.
In order to meter the flow of such fluid and solid mixtures, standard single-phase flowmeters such as orifice plates, venturi meters or vibrating tube mass flowmeters are presently used and the liquid flowing in the wet gas, or the solid present in the gas or liquid, is effectively a contaminant which modifies the behavior or calibration of the device from its normal behavior in single-phase flows. A correction is carried out.
The correction of the reading of a gas meter used in wet gas or pneumatic conveying depends on the quantity of liquid or solid flowing, and also on the distribution of the liquid or solid in the pipe and the slip velocity. Similarly, the correction of the reading of a liquid meter used in solids conveying depends on the quantity of liquid flowing, and also on the distribution of the liquid in the pipe and the slip velocity
Since normally the distribution and velocity of the distributed liquid or solid is not measured, assumptions are made which increase the uncertainty of the indicated value of flowrate. Because of the wide range of multiphase industrial flows and their substantial commercial importance, the resultant potential financial uncertainty is very significant.
Many of the state-of-the art methods for measuring flowrate include a differential pressure measurement. Examples include orifice plates and venturis. These methods and the meters that embody them are widespread and relatively low cost but suffer from significant limitations including:                differential pressure devices may be used which are, by their nature, delicate relative to the level of pressure in the pipe and can be destroyed by pressure pulses in the flow;        using robust absolute pressure measurement sensors limits the resolution of the measurement because they use a method which involves taking the difference of two values each of which is large relative to the absolute value of the difference; and        small discontinuities in the tubes (to provide pressure ports) linking the flow to the sensors which may add substantial error which is not appreciated and compensated for by the user.        
Other multiphase flows in addition to the wet gas flows described above contain a mixture of materials, variously gas, liquid, divided solids and combinations thereof, and there are a variety of flow regimes or patterns that occur. For example, a gas may flow smoothly over the top of a liquid layer in a horizontal pipe, large bubbles may fill the pipe intermittently, slurries or solids may move in waves, or the flow may even plug and stop completely. Many other flow patterns may occur in diverse mixtures. Each of these flow regimes or patterns may have commercially or technically important implications, for example the energy used to transport the fluids may vary with flow regime, the mixing between the phases may be important in the process and will vary with flow regime, it may be important that certain of the phases do not touch the internal surface of the pipe, some flow regimes may lead to more erosion or deposition than others, and different flow regimes may influence the output from conventional flow sensors in such a way that measurements made of the flow are not correct.
It is envisaged that the invention will be applicable to many or all of the above flow types and others. The described specific embodiments are illustrative to provide an understanding of the invention and should not be taken to limit the invention the depicted applications or particular arrangements.
A number of physical principles have been used to visualize internal flow processes in industrial pipes or vessels, these are referred to, in general, as flow imaging or process imaging. Such methods include the measurement of electrical capacitance, electrical conductivity, x-rays, gamma-rays, nuclear magnetic resonance, magnetic fields or nucleonic emission of particles and a determination of how they vary across the length and breadth of the flow.
Many of the “imaging” techniques have been applied in research tests and pilot-scale plants but few are in wide-scale use in industrial applications. For example: CN2826373 (Y) discloses image pick-up tracking and detecting system, GB2212903 (A) describes x-ray stereoscopic imaging system, CN1344929 (A) discloses resistance chromatographic instrument, CN102410974 (A) discloses pulse laser sources, WO2012031292 (A1) discloses ultrasonic scanning, WO2012100385 (A1) discloses a gamma ray imaging device, KR100815210 (B1) discloses a 3D particle mage velocity meter, JP2004212117 (A) discloses X-ray. US2011109308 (A1) and US2012174684 (A1) describe magnetic resonance and CN102364046 (A) discloses light intensity modulation type optical fiber sensors.
Normally such imaging techniques are used to show a cross-section of the pipe normal to the flow direction or a longitudinal section along the flow direction but more recent developments have led to 3-dimensional imaging of the body of the flows. For example: JP2004333237 (A) describes a method to reconfigure the visualization image of a mixed flow in a fluid transport pipe by process tomography and CN2695964 (Y) describes discloses an oil-gas two-phase flow investigating device based-on capacitive chromatographic imaging system.
Flow imaging techniques such as ECT (Electrical Capacitance Tomography) have been developed into working flowmeters for certain applications and other of the techniques are used as part of the technology of multiphase flow metering. GB2390683 (B) describes a flow meter based on electrical capacitance tomography wherein image data sets representing concentration are used to calculate flowrates of multiphase flows and for example Hunt, Pendleton and Byars in 2004 described in (2004) “Non-intrusive measurement of volume and mass using electrical capacitance tomography” ESDA 2004-58398, 7th biennial ASME conference, Jul. 19-22 2004, Manchester UK, a method of estimating volume and mass of solid objects using ECT, while Hunt Pendleton and Ladam in 2004 in “Visualisation of two-phase gas-liquid pipe flow using electrical capacitance tomography” ESDA 2004-58396, 7th biennial ASME conference, Jul. 19-22 2004, Manchester UK, described measurements of flow structures in mixtures of oil and gas. In all three of these references, as in other publications, the ECT sensors are calibrated so as to measure the fraction of volume that each phase is present in any given volume of the pipe. This fraction is known as the “volume fraction” or “holdup”.
The difficulties of implementing flow or process imaging in industrial applications include the cost—such devices often cost ten times or more the cost of standard flow meters or other sensors; the danger and complexity of implementing nucleonic measurements in standard industrial processes; and the practical difficulty of making complex measurements of this kind in pipes at high pressure and temperature. In addition, knowledge of the volume fraction may not be sufficient or appropriate. The inventor of the present application made the realization that knowledge of the density distribution in the pipe may be more helpful.
A particular problem arises in propellant/fuel or other fluid storage vessels in space vehicles and that is determining how much of the propellant/fuel remains within the vessel and or associated pipework. As the fluid is used under low or zero gravity conditions, it disperses as droplets in a mixture of sizes with empty space between. This makes it difficult using traditional methods to determine the volume remaining and assumptions based on “burn time” consumption after first filling are used to provide an estimate. As will be appreciated, this can lead to tanks being filled with more propellant/fuel than is actually needed to provide a “safety margin”. Given the very high costs per kilogram of launching vehicles or satellites into space, the cost of launching into space are greatly added to by the excess fluid/propellant/fuel.